The present invention relates to an improvement in the single crystal growing of a rare earth-gallium garnet by the so-called Czochralski method in the preparation of a single crystal boule having a relatively large, e.g. 50 mm or larger, diameter and outstandingly free from crystal defects.
As is well known, single crystals of a rare earth-gallium garnet having a composition expressed by the formula R.sub.3 Ga.sub.5 O.sub.12, in which R is a rare earth element such as gadolinium, samarium, neodymium and the like, are useful materials in the technology of electronics, for example, as a substrate of the magnetic bubble domain memory. It is very important that the substrate for the magnetic bubble domain memory should be as free as possible from any crystal defects since the defects in the substrate crystal necessarily result in the defect in the epitaxial surface film of the rare earth-iron garnet formed on the surface of the substrate by the method of the liquid phase epitaxial growing.
Single crystal boules of the rare earth-gallium garnet are usually prepared by the method of so-called Czochralski crystal growing in which a mixture of gallium oxide and the rare earth oxide, e.g. gadolinium oxide, samarium oxide, neodymium oxide and the like, is melted in a crucible made of a refractory metal such as iridium and a seed crystal of the garnet is contacted to the surface of the melt and then pulled up gradually with simultaneous rotation around the vertical axis so that the single crystal grows from the melt on the lower end of the seed crystal.
It is known that, in the above described process of the single crystal growing by the Czochralski method, dislocations and lattice distortion are produced in the single crystal boule by the presence of fine particles of the lower oxide formed by the decomposition of part of the starting oxides and the iridium metal formed from the crucible in the melt at the extremely high temperature for crystal growing which unavoidably are taken up into the single crystal grown from the melt as inclusions. When such a defective single crystal boule is used as the base material for the substrates, the yield of acceptable products of the magnetic bubble domain memory is naturally decreased.
Needless to say, various attempts have been made to decrease the crystal defects in the single crystal boules of the rare earth-gallium garnets including a method in which the melt of the oxide mixture is kept for a length of time of 1 to 10 hours under an atmosphere of which the partial pressure of oxygen is from 3.8 to 19.4 mmHg before the melt is seeded with a seed crystal (see, for example, U.S. Pat. No. 3,723,599), a method in which pulling-up of a single crystal by the Czochralski method is performed under an ozone-containing atmosphere formed by the ozonization treatment of a gas containing from 0.25 to 1.5% by volume of oxygen (see, for example, Japanese Patent Publication No. 54-1278), a method in which the Czochralski single crystal growing is performed under an atmosphere of nitrogen gas containing 0.5 to 3% by volume of oxygen (see, for example, Japanese Patent Kokai No. 55-136200) and a method in which the iridium crucible is first heated in an atmosphere containing at least 1% by volume of oxygen to oxidize the surface followed by dissipation and removal of the surface oxide of iridium by heating at 1500.degree. C. or higher in an inert atmosphere (see, for example, Japanese Patent Publication No. 51-35600).
These prior art methods are, however, far from sufficient to afford a complete solution to the above mentioned problems since the crystal growing in each of these methods is performed under an atmosphere containing oxygen or oxidizing gas so that the oxidation of iridium as the material of the crucible cannot be completely prevented. Moreover, the amounts of the undesirable inclusion in the oxide melt in these prior art methods are susceptible even to a very slight change in the conditions such as the composition of the starting oxide mixture, composition of the atmospheric gas, structure of the heat-insulating muffles and the like. This problem is more and more serious as the diameter of the single crystal boule is increased in compliance with to the recent trend of the technology requiring single crystal boules of larger and larger diameter in order to improve the efficiency for the preparation of the substrates for the magnetic bubble domain memories.